This invention relates to optical diffraction and reflection gratings, and more particularly, this invention relates to Bragg gratings.
Bragg gratings and similar fiber optic and other optical grating structures are produced in glass, plastic or silicon to spread out an optical spectrum or other radiation. These gratings usually consist of narrow, parallel slits or narrow, parallel, reflecting surfaces that break-up waves as they emerge.
As is well known, light of all wavelengths is scattered at all angles. At some angles, however, light adds constructively at one wavelength, while other wavelengths add destructively (or interfere with each other), reducing the light intensity to zero or close to zero. In those ranges of angles where the grating spreads out a spectrum, there can be a gradual change in wavelength of the angle. With multiple grooves formed in a grating, light is concentrated in particular directions, and can be used as optical filters with other similar optical devices.
One commonly used optical grating is a Bragg grating used as a periodic grating, a chirped grating, a distributed feedback or distributed Bragg reflector grating (DFF or DBR), such as with laser, and a Fabry-Perot Etalon grating for a ring resonator as designed for use with add/drop multiplexers and similar optical devices. A Bragg grating is the optical equivalent of a surface acoustic wave (SAW) device. By having a tuned grating, there can be some compensation for dispersion conditions. Some optical filters use Bragg gratings that are tuned during fabrication, temperature tuned, or compression/strained tuned.
Prior art solutions for tuning gratings using temperature or compression/strain methods have a limited tuning range of typically only tens of nanometers maximum with a slow operation of tuning. As is known, the temperature and strain changes on Bragg deflection and change are set forth as:
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Also, multiple configurations are typically not possible in a single prior art device.
It is therefore an object of the present invention to provide tunable optical gratings that do not involve the tuning of the gratings using temperature or strain changes.
It is still another object of the present invention to provide a tunable optical grating where the grating profile can be controlled over a wide range, such as in hundreds of nanometers.
It is still another object of the present invention to provide a tunable optical grating having multiple configurations possible with a single device.
The present invention is advantageous and provides a tunable optical grating having a plurality of grating structures that are contained within an optical transmission path. A microelectromechanical (MEMS) actuator is operatively connected to each grating structure for changing the separation between the grating structures and tuning the grating to a desired wavelength selectivity.
In one aspect of the present invention, the grating structures form a Bragg grating and are periodic gratings. In yet another aspect of the present invention, the grating structures form a chirped grating. In still another aspect of the present invention, the grating structures can be a distributed feedback grating, distributed Bragg reflector grating, or a Fabry-Perot Etalon add/drop grating structure.
The Bragg or other grating can be formed on a silicon MEMS substrate. Formed MEMS actuators operatively connect each grating structure. The MEMS actuators can be photolithographically formed on the MEMS substrate or by other MEMS fabrication techniques, known to those skilled in the art. In yet another aspect of the present invention, the MEMS actuators can each comprise a flat, single layer silicon membrane structure.
In another aspect of the present invention, the MEMS actuators can each comprise at least one anchor support, and an arm member operatively connected to a grating structure, supported by the anchor support, and moveable therewith for moving the grating structure relative to another grating structure. The MEMS actuators also can comprise a hinged plate actuator operatively connected to each grating structure.
A tunable grating apparatus of the present invention can also comprise an optical waveguide defining an input port through which an optical signal is formed, such as a multi-wavelength optical signal, which passes through the grating structures. An optical waveguide can define an output port for receiving the optical signal from the grating structures. A collimating lens can be operatively connected to the input port to form a collimated optical signal. A converging lens can be operatively connected to the output port to converge the optical signal, all by techniques using lenses known to those skilled in the art.
In yet another aspect of the present invention, a tunable, add/drop optical network element includes an input port for receiving a multi-wavelength, optical signal and passing the optical signal along an optical transmission path. An output port receives the optical signal along the optical transmission path and passes the multi-wavelength optical signal with added or dropped optical signal channel components. An optical add/drop element is contained within the optical transmission path and includes a plurality of Bragg grating structures contained within the optical transmission path and forming a Bragg grating for receiving the optical signal and passing and/or reflecting optical signal channel components of a desired wavelength. A microelectromechanical (MEMS) actuator is operatively connected to each Bragg grating structure for changing the separation between the Bragg grating structures and tuning the Bragg grating to a desired wavelength selectivity.
In yet another aspect of the present invention, add and drop ports are operatively connected to the optical add/drop element, where optical signal channel components of desired wavelength are added and dropped. The Bragg grating structures are preferably configurable to be responsive to different optical signal channel components.
In yet another aspect of the present invention, a tunable laser and filter apparatus includes a semiconductor substrate and a laser structure formed on the semiconductor substrate. The laser structure includes an active layer and a plurality of Bragg grating structures formed along the active layer to form a Bragg grating and provide optical reflections at a desired Bragg wavelength. A microelectromechanical (MEMS) actuator is operatively connected to each Bragg grating structure for changing the separation between the Bragg grating structures and tuning the Bragg grating to a desired wavelength selectivity and limiting the laser output to a selected narrow band mode.